getting worried

Kenny
Hey new parents. Getting worried here. My little guy is 7.5 months now and he has no interest at all in sitting up unassisted. I have been hearing all around that by this point he should be almost crawling. He will only sit up for a few seconds with me supporting his back. 
He will sit up on my lap. But he immediately stretches out, or arches his back. I know every baby is different. Just getting worried. 😩